In a typical cellular network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS) or base station, which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the base station at a base station site or an antenna site in case the antenna and the base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. The user equipment transmits data over the radio interface to the base station in Uplink (UL) transmissions and the base station transmits data over an air or radio interface to the user equipment in Downlink (DL) transmissions.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (VVCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for e.g. third generation networks and further generations, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a radio network controller are distributed between the base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising base stations without reporting to RNCs.
A cellular network typically includes some areas with high traffic, e.g., a high concentration of users. In those areas it may be desirable to deploy additional capacity to ensure user satisfaction. The added capacity may be in the form of additional macro base stations, e.g., more NodeBs in WCDMA terminology, and/or in the form of lower output power base stations. A lower output power base station covers a smaller area in order to concentrate the capacity boost in a smaller area. Examples include micro, pico, home base stations, relays, etc. Often, there are also areas with poor coverage where there is a need for coverage expansion, and one way to address these coverage issues is to deploy a lower output power base station, also called low output power node, to provide the coverage boost in a small area. A benefit with low power nodes in these situations is that their impact on the macro network is reduced, e.g., a smaller area in the macro network experiences interference.
A network deployment that uses both higher power macro nodes and low power nodes is referred to here as a heterogeneous network or “HetNet.” Multiple layers in a HetNet are illustrated in the example of FIG. 1. A higher power macro base station, the high tower, provides a wide area coverage called a macro cell, and low power nodes, the shorter structures, provide small area capacity/coverage in smaller cells, dotted areas. In this example, pico base stations and pico cells, relays and relay cells, grey area, and home base stations sometimes called femto base stations and femto cells, striped areas, are shown. Although FIG. 1 shows clusters of femto cells, single femto cell deployments may also be used.
Because cells in a HetNet typically operate with different pilot power levels, there may be imbalances between the radio UL and the radio DL in the network. Cells are typically selected by UEs based on their measurements of the received signal strength of downlink transmissions from those cells, with UEs being served by the best downlink cell alternative. However, the uplink quality depends mainly on the distance between the user equipment and the serving base station site and is generally independent of the serving cell's downlink pilot power. As a result of a UE's serving cell/base station selection being based on downlink pilot signals, UEs may have a better uplink signal quality to a non-serving cell. Two examples of use cases for heterogeneous network deployment include coverage holes and capacity enhancement for localized traffic hotspots.
In WCDMA systems, a user equipment in a soft handover (SHO) is power controlled by the best uplink cell. If the cell with the best UL is a non-serving cell, one problem is how to ensure that important control information can be reliably received at the serving macro base station. The problem of weak links becomes particularly pronounced whenever the imbalance between the best UL and DL may become large, such as heterogeneous network or multi-flow operation.
SHO, also referred to as macro diversity, and fast closed-loop power control are essential features of WCDMA and Enhanced Uplink (EUL). FIG. 2 illustrates a traditional HSPA deployment scenario with two nodes having a similar transmit power level. One macro node providing a serving cell and one macro node providing a non-serving cell. Ideally, a user equipment moving from the serving cell towards the non-serving cell would enter a SHO region at point A (Event 1a), at point B (Event 1d) a serving cell change would occur, i.e. non-serving becomes serving and vice versa, and at point C (Event 1b) the user equipment would leave the SHO region. It is the radio network controller that is in control of reconfigurations, which implies rather long delays for e.g. performing a cell change. During SHO, the user equipment is power-controlled by the best uplink cell. Since the nodes have roughly the same transmit power, the optimal DL and UL handover cell borders will coincide, i.e. the path loss from the user equipment to the two nodes will be equal at point B and equal DL Rx power border is at point B. Hence, in an ideal setting and from a static, long-term fading, point of view, the serving cell would always correspond to the best uplink. However, due to practical implementation issues, e.g. reconfiguration delays, and fast fading, the user equipment might be power controlled by the non-serving cell during SHO. In such case there might be problems to receive essential control channel information in the serving cell due to the weaker link between the serving cell and UE. For example, the uplink High Speed—Dedicated Physical Control Channel (HS-DPCCH) and uplink scheduling information need to be received in the serving cell. For heterogeneous networks, other factors make the imbalance more pronounced. This may reduce the performance of the cellular network.
Possible solutions include increasing uplink gain factors using of radio resource control (RRC) signaling and sending repeated transmissions, e.g. based on Hybrid Automatic Repeat Request (HARQ) until a transmission is successfully received.
Since Low Power Nodes (LPN) and macro NodeBs in a heterogeneous network have different transmit powers, the uplink (UL) and downlink (DL) cell borders do not necessarily coincide. An example of this is when a user equipment has a smaller path loss to the LPN, while the strongest received power is from the macro NodeB. In such a scenario, the UL is better served by the LPN while the DL is provided by the serving macro NodeB. This is shown in FIG. 3. The region between the equal path loss border and equal downlink received power, e.g. (paging channel) Common Pilot Channel (CPICH) receive power from macro node denoted Power CPICH1 is equal receive power from LPN denoted Power CPICH2, border is referred to as imbalance region.
In this imbalance region, some fundamental problems may be encountered. For example, a user equipment in position A would have the Macro Node as the serving cell, but be power controlled towards the LPN. Due to the UL-DL imbalance the UL towards the serving macro node would be very weak, which means that important control information, such as scheduling information or HS-DPCCH, might not be reliably decoded in the serving cell but only received at the LPN. Furthermore, a user equipment in position B would have the Macro Node as the serving cell, and also be power controlled towards the macro. Due to the UL-DL imbalance, the user equipment would cause excessive interference in the LPN node. Furthermore, in this scenario we cannot fully utilize the benefits of macro offloading towards the LPN. One way of improving these problems is to utilize the available radio network controller based cell selection offset parameters, shown in FIG. 4. By tuning or adjusting a Cell Individual Offset (CIO) parameter, the handover border may be shifted towards a more optimal UL border. Similarly, the WCDMA IN_RANGE and OUT_RANGE parameters may be adjusted in order to extend the SHO region. The effect of these adjustments is illustrated in FIG. 4. The FIG. 4 illustrates how the CIO may be used to move the handover area closer to the Macro node, and that the SHO area may be extended compared to previous FIG. 3.
These adjustments are beneficial from a system performance point of view, but some difficulties remain:                Scenario 1—A user equipment in position A may experience a poor DL from the non-serving LPN. This may complicate a reliable detection of UL related DL channels, e.g. E-DCH-HARQ Indicator Channel (E-HICH) and Fractional Dedicated Physical Channel (F-DPCH) from the LPN. E-DCH stands for Enhanced Dedicated Channel.        Scenario 2—A user equipment in position B has the macro cell as serving cell but is, in general, power controlled towards the LPN. Hence, the uplink signal towards the serving cell might be weak and thereby complicate a reliable reception of control channel information at the serving cell.        Scenario 3—A user equipment in position C is served by the LPN. However, its DL might be poor and thereby complicate a reliable reception of control information, such as High Speed Shared Control Channel (HS-SCCH) and E-DCH absolute grant channel (E-AGCH).        Scenario 4—A user equipment in position D might experience a poor UL towards the non-serving macro cell and thereby complicate the uplink reception at the macro cell.        
To maximize the potential gains provided by range extension, the problems associated with the different scenarios above need to be solved. Thus, there is a need to both optimize the system performance and improve the link quality for UEs experiencing significant degradation in the UL or DL. In addition, there is a need to provide reliable reception of UL control information, while at the same time minimizing interference from one or more data channels, e.g. the Enhanced Dedicated Channel Dedicated Physical Data Channel (E-DPDCH), when the UL communication link, e.g. the E-DPCCH, is weak to avoid a reduced performance of the cellular network.